In a digital modulation method used for recent wireless communication such as a mobile phone or a wireless LAN (Local Area Network), a modulation format of QPSK (Quadrature Phase Shift Keying) or multivalued QAM (Quadrature Amplitude Modulation) is employed.
In such a modulation format, generally, the locus of a signal is accompanied by amplitude modulation during a change between systems. This causes, in the case of a high-frequency modulation signal that is superimposed on a carrier signal of a microwave band, a change in amplitude (envelope) of the signal over time. In this case, the ratio of peak power to average power of the high-frequency modulation signal is referred to as PAPR (Peak-to-Average Power Ratio).
When a signal having large PAPR is amplified, to secure high linearity, sufficiently large power must be supplied from a power source to an amplifier so as to prevent a waveform from being distorted with respect to the peak power. In other words, the amplifier must be operated with a margin (back-off) in a region of power sufficiently lower than saturated power limited by a power supply voltage. Generally, in a linear amplifier operated with a grade A or a grade B, power efficiency is highest near its saturated output power. Thus, when the amplifier is operated in a large back-off region, average efficiency is lower.
In an OFDM (Orthogonal Frequency Division Multiplexing) method using a multicarrier employed in a next-generation mobile phone, wireless LAN, or digital television broadcasting, PAPR tends to be very large, and the average efficiency of the amplifier is much lower. Thus, as characteristics of the amplifier, desirably, high efficiency is provided even in the large back-off power region.
As a method for amplifying a signal to high efficiency over a wide dynamic range in the large back-off power region, there is known a transmission method such as Envelope Elimination and Restoration (EER) or Envelope Tracking (ET).
In the EER method, first, an input modulation signal is decomposed into its phase component and its amplitude component. The phase component is input to a power amplifier at constant amplitude while phase modulation information is maintained. In this case, the power amplifier is always operated near saturation where efficiency is highest. On the other hand, in the case of the amplitude component, the output voltage of the power supply device is changed according to amplitude modulation information, and this is used as power for the amplifier.
Thus operated, the power amplifier operates as a multiplier, the phase component and the amplitude component of the modulation signal are synthesized, and an output modulation signal amplified with high efficiency irrespective of back-off is obtained.
In the ET method, a configuration in which the amplitude component of the input modulation signal changes the output voltage of the power supply device according to amplitude modulation information, and this is used as power for the amplifier is similar to that of the EER method. A difference is that while in the EER method, the amplifier is operated in the saturated manner by inputting only the phase modulation signal of constant amplitude to the amplifier, in the ET method, the amplifier is operated in the linear manner by directly inputting the input modulation signal including both amplitude modulation and phase modulation to the amplifier.
In this case, efficiency is lower than that in the EER method because of the linear operation of the amplifier. However, since only minimum necessary power is supplied to the amplifier according to the size of the amplitude of the input modulation signal, higher power efficiency can still be obtained compared with a case where the amplifier is used at a constant voltage irrespective of amplitude. In the ET method, a timing margin for synthesizing the amplitude component and the phase component is weakened, thus providing an advantage of easier realization than the EER method.
The modulation power supply device used in the EER method or the ET method must be a voltage source that can accurately and highly efficiently change the output voltage with low noise according to the amplitude component of the input modulation signal. It is because in the recent wireless communication method such as a mobile phone using digital modulation, ACPR (Adjacent Channel Leakage Power Ratio) or EVM (Error Vector magnitude) representing a modulation error must be reduced to a fixed value or lower to meet the standard.
When the output voltage of the power supply device is not linear with respect to the input amplitude signal, ACPR or EVM deteriorates due to mutual modulation distortion. When the noise of the power supply enters into the output of the amplifier, ACPR also deteriorates.
It is said that in the EER method or the ET method, the response band (speed) of the power supply device must be at least twice as large as or more than that (speed) of the modulation signal. For example, according to the WCDMA (Wideband Code Division Multiple Access) standard of the mobile phone, a modulation band is about 5 MHz. According to the IEEE 801.11 a/g standard of the wireless LAN, a modulation band is about 20 MHz. In the general power supply device of a switching converter configuration, it is difficult to output the modulation signal of such a wide band.
Related to the foregoing, Nonpatent Literature 1 (upper stage of FIG. 2) discloses, to achieve a highly efficient and high-quality voltage source, two basic configurations of hybrid voltage sources that combine highly efficient switching amplifiers with highly accurate linear amplifiers.
FIG. 1 is a diagram showing, as one of the two hybrid voltage sources disclosed in Nonpatent Literature 1, the configuration of a power supply device having a voltage modulation function according to a first related technology. In the drawing, A1 is an equivalent circuit.
In the power supply device shown in FIG. 1, switching amplifier 2 operating as a current source and linear amplifier 3 operating as a voltage source are connected in parallel. With this configuration, highly accurate linear amplifier 3 plays the role of correcting output voltage Vout to be equal to reference signal Vref. On the other hand, switching amplifier 2 detects output current Ic from linear amplifier 3. Then, based on the result, control signal generation unit 4 controls switching elements 21 and 22.
Through such an operation, switching amplifier 2 operates as a current source, and the most part of power supplied to load 1 is supplied from highly efficient switching amplifier 2. Accordingly, since linear amplifier 3 that is high in accuracy but low in efficiency consumes only power necessary for eliminating ripples included in output voltage Vout, a voltage source that is high in both accuracy and efficiency can be achieved.
FIG. 2 is a diagram showing, as the other of the two hybrid voltage sources disclosed in Nonpatent Literature 1, the configuration of a power supply device having a voltage modulation function according to a second related technology. In the drawing, A2 is an equivalent circuit.
In the power supply device shown in FIG. 2, switching amplifier 2 and linear amplifier 3 are connected in series. With this configuration, highly accurate linear amplifier 3 plays the role of applying feedback to and correcting output voltage Vout to be equal to reference signal Vref. On the other hand, switching amplifier 2 applies feedback to output voltage Vm to be almost equal to reference voltage Vref (or output voltage Vout obtained by linearly scaling the same), and control signal generation unit 4 controls switching elements 21 and 22.
Output voltage Vc of linear amplifier 3 is added to this in series, for example, by transformer 35. Through such an operation, the most power that is supplied to load 1 is supplied from highly efficient switching amplifier 2. Accordingly, since linear amplifier 3 that is high in accuracy but low in efficiency consumes only power necessary for eliminating ripples included in the output voltage, a voltage source that is high in both accuracy and efficiency can be achieved.
The configuration of the hybrid voltage source combining the switching amplifier with the linear amplifier is classified into one of those shown in FIGS. 1 and 2.
Nonpatent Literature 2 (FIG. 6) discloses an amplifier where the configuration of the hybrid voltage source shown in FIG. 1 is applied to the power supply device of the ET method.
FIG. 3 is a block diagram showing the configuration of a transmission device that uses the power supply device shown in FIG. 1.
In the transmission device shown in FIG. 3, amplitude component 9 of the input modulation signal is input to a portion equivalent to reference voltage Vref shown in FIG. 1, and highly efficient and wide-band modulation voltage 11 that is obtained is supplied as power for power amplifier 1. Hereinafter, the specific operation of the transmission device shown in FIG. 3 will be described by using an operating waveform.
FIGS. 4a to 4c are diagrams each showing the operating waveform of the power supply device shown in FIG. 1.
First, amplitude signal 9 is input to linear amplifier 3 of a voltage follower including operational amplifier 31. In this case, as input amplitude signal 9, the envelope of a WCDMA downlink signal is used (refer to waveform 9 shown in FIG. 4a).
Then, output current (operational amplifier current) Ic of linear amplifier 3 is converted into a voltage at current detection resistor 7 to be input to hysteresis comparator 51 of control signal generation unit 4. In this case, hysteresis comparator 51 selects signal polarities to be High when current flows out from linear amplifier 3 (Ic>0) and Low when current flows in (Ic<0). Accordingly, a pulse width modulation signal is output according to the intensity of input amplitude signal 9. This pulse width modulation signal is used as the control signal of switching element 21 typically including MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
Switching element 21 constitutes a switching converter with diode 22. When control signal 50 from hysteresis comparator 51 is High, switching element 21 is turned ON (conductive state), and current flows from power source Vcc 1 to load 1. In this case, switching voltage Vsw is set to Vcc 1 (set to 15 V) (refer to waveform 10 shown in FIG. 4c). The current from switching element 21 is passed through inductor 23 (set to 0.6 uH) to be integrated, thus becoming switching current Im where a switching frequency component has been eliminated.
At output terminal Vout, Ic=Iout−Im is established. Thus, when switching current Im of switching amplifier 2 is excessive with respect to output current lout flowing from output terminal Vout to power amplifier 1, operational amplifier current Ic of linear amplifier 3 reversely flows (Ic<0), and starts to flow in a direction into operational amplifier 3.
As a result, the polarity of control signal 50 of hysteresis comparator 51 is reversed from High to Low, and switching element 21 is turned from ON to OFF (nonconductive state). At this time, to maintain the current flowing through inductor 23, current Im flows from ground (GND) to load (power amplifier) 1 via diode 22. Cathode potential Vsw of diode 22 is set to 0 V (refer to waveform 10 shown in FIG. 4c).
By repeating the aforementioned switching operation, switching current Im is supplied to load 1 alternately from Vcc 1 and GND (refer to waveform 13 shown in FIG. 4b).
Switching current Im 13 includes an error component generated by switching. However, a voltage is corrected by linear amplifier 3, and output signal 11 (refer to waveform 11 shown in FIG. 4c) is supplied to amplifier 1 by accurately reproducing and amplifying the waveform of input signal 9.
In the series of operations, the current (refer to waveform 14 shown in FIG. 4b) flowing through operational amplifier 31 that is low in efficiency is only an error component, and thus the power consumption of linear amplifier 3 is reduced. Since the most the input signal is amplified by highly efficient switching unit 2, the efficiency of the power supply device can be increased.
By performing the aforementioned EER or ET operation using output voltage Vout thus obtained as power for power amplifier 1, only minimum necessary power is supplied from the power supply device according to the amplitude of the input modulation signal. As a result, power amplifier 1 can always operate near saturation of high efficiency, and the power efficiency of the entire transmitter system including the power supply device and the power amplifier can be increased.
Related to the foregoing, Patent Literature 1 discloses an amplification circuit for amplifying an input signal, including a voltage source that operates with a first bandwidth and first power efficiency and generates a first signal having a voltage changed with the input signal, a current source that operates with a second bandwidth not wider than the first bandwidth and second power efficiency that is higher than the first power efficiency and generates a second signal having current changed with current in the first signal, and a load which is coupled in parallel between the voltage source and the current source and to which the first and second signals are applied.
Patent Literature 2 discloses an amplification device for amplifying the amplitude component of an input signal including the amplitude component and a phase component to synthesize it with the phase component, thereby generating an output signal obtained by amplifying the input signal, including a pulse modulation unit that generates a pulse modulation signal by subjecting the amplitude component of the input signal to pulse modulation to amplify it, a low-pass filter that generates an amplified amplitude signal by filtering the pulse modulation signal from the pulse modulation unit to amplify the amplitude component, an error correction unit that generates a corrected amplitude signal by correcting an error included in the amplified amplitude signal from the low-pass filter by using the amplitude component of the input signal, and a synthesis unit that generates an output signal by synthesizing the corrected amplitude signal from the error correction unit with the phase component of the input signal.